Merry Christmas or Merry Xmas or Merry X'mas?

I remember one of the fathers in my high school told us that we should always write Christmas in full, because Christmas is to remember Jesus Christ's birth. If we really want to use short form, write Xmas, not X'mas, because X stands for Christ, and there is no need to add the apostrophe in between. We see so many X'mas nowadays...

Merry Christmas!